A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
A potential problem in such systems is that particles of the fuel material tend to be ejected along with the radiation, and can travel at high or low velocities through the apparatus. Where these particles contaminate optical surfaces such as the mirror lenses or the patterning device (e.g. reticle), performance of the apparatus is degraded. U.S. Pat. No. 6,781,673 by ASML proposes electrostatic deflection to protect a patterning device (e.g. reticle), and the same principles can be applied in protecting other components and spaces of the apparatus. U.S. Pat. No. 6,781,673 proposes charging of particle using photoelectric effect of the EUV beam itself, which yields a positive charge on the tin particles.
Depending on the situation, the photoelectric charging may not be enough to deflect all the unwanted particles. A further potential problem may arise in trying to apply this technique in the hydrogen environment mentioned above. Where gas (H2) is present, the EUV radiation pulses may generate a conductive hydrogen plasma. When this H2 plasma (generated by the EUV beam) is present in the region between the capacitor plates, the applied E-field will be screened by plasma, and may not deflect the particles. Additionally, the plasma will gradually apply a negative charge to the particles, thereby erasing the positive charge of the photoelectric effect.
United States Patent Application Publication No. 2007/0001126 describes a ‘foil trap’ arrangement for preventing contaminant particles from reaching from a source plasma to a collector optical element of an EUV source apparatus. Electrostatic fields are created between adjacent blades of the foil trap, which accelerate dissipation of the plasma, permitting electrostatic deflection of particles to occur. One embodiment of a foil trap in United States Patent Application Publication No. 2007/0001126 is arranged in two stages, spaced along the path of the radiation and contamination. In a first stage, plasma coming from the source is dissipated by an electric field between blades. In a second stage, no electric field is applied, and the particles are slowed and repelled only by a counter-flowing gas. By improving dissipation of the plasma, it is said that the gas flow can be reduced as compared with prior foil traps.
Another prior publication, United States Patent Application Publication No. 2008/0083887 (Komatsu) proposes to use a strong magnetic field to deflect debris particles around a laser-produced plasma EUV source. In order to ionize particles so that the particles will be trapped by the magnetic field, microwave radiation (electromagnetic radiation in the GHz range) is applied, together with a supply of free electrons, to induce electron cyclotron resonance. In view of additional potential problems consequent on the use of microwaves, measures are disclosed for applying the microwave radiation only in pulses, synchronized with the source laser. Assuming also that the magnetic field requires a superconducting electromagnet to generate it, the entire arrangement may become relatively cumbersome and expensive.